Global Navigation Satellite Systems (GNSS) include the Global Positioning System (GPS), the GLONASS system, the proposed Galileo system, and the proposed Beidou (Compass) system.
The Global Positioning System completed its original design goals when it attained full operational capability in 1995. Technical advances and new demands on the system have since led to a modernization effort. The GPS modernization project involves new ground stations and new satellites, with additional navigation signals and improved accuracy and availability. The first GPS satellite with three-frequency capability including the new L5 frequency, GPS Block IIF-1, is expected to be launched in the summer of 2009. The new civilian-use L5 signal is expected to improve signal structure for enhanced performance, with higher transmission power and wider bandwidth than the L1 and L2C signals to better manage interference than with L2. Launch of additional three-frequency GPS satellites is planned, with a full three-frequency constellation probably available only 5-7 years later.
The European Galileo satellite system will have similar three-frequency capabilities, but may not provide them all free-to-air. To date only two Galileo validation element satellites GIOVE-A and GIOVE-B have been launched. Further, the Galileo launch schedule is lagging behind the original plan. The Chinese Compass system is in the early stages of testing, but may offer three-frequency capabilities when it eventually becomes operational. The Russian GLONASS system is also expected to have three-frequency capabilities at some time in the future.
There will be a transitional period during which a subset of the GNSS satellite constellation will have three-frequency capabilities, while the remainder will continue to broadcast on just two frequencies.
U.S. Pat. No. 7,432,853, VOLLATH U., “Ambiguity Estimation of GNSS Signals for Three or more Carriers” presents a distributed filtering schemes which efficiently deliver ambiguity estimates for two, three or more carrier signals, and addresses to some of the issues raised with mixed constellations of two and three-or-more frequency satellites.
Improved methods and apparatus for processing GNSS signals are desired, particularly to improve ambiguity estimation of GNSS signals from satellites having mixed numbers of carriers.